This invention relates to a method of expanding a metal tube into engagement with a surrounding coaxial tubular metal member by means of an explosive charge. The method may be operated so that the expanded portion of tube becomes either mechanically engaged with, or explosively bonded to, the surrounding metal member, and is particularly advantageous for joining large tubular metal elements. One especially useful commercial application of the invention is in attaching steel coupling members to the ends of aluminium pipe lengths for assembly into composite oil-well drill pipes.
Oil-well drill pipes are normally made up of steel pipe segments at each end of which is a steel coupling. These couplings are screwed together to lengthen the drill pipe as drilling depth increases thereby making up a composite length of the drill pipe.
There are advantages in using aluminium pipe which will more easily bend. This allows greater freedom and increased capability in directional drilling. It is desirable, however, to use steel to provide strong coupling components on each end of the drill pipe.
As steel and aluminium are incompatible for fusion welding the two components cannot be joined by this method. Screwing the components together is also impracticable since screw threads reduce the aluminium wall section to intolerable levels unless the wall thickness of the aluminium is increased by local upsetting of the tube end during manufacture and this is expensive. Shrink fitting is an alternative procedure which has had only very limited success due to a high incidence of joint failure.
This invention provides a novel method of attaching the steel coupling members by means of explosive expansion of the aluminium tube into a suitable profiled bore of the steel coupling.
The use of explosives to expand a tubular metal component into the bore of a second metal component to form either a mechanical or explosively bonded joint is well known. The bore configuration of the outer component or the outside diameter of the inner component must be specifically contoured for fabricating an explosively bonded joint and/or the detonation rate of the explosive must be tightly controlled. The explosive force must be considerable to achieve an explosive bond and this requires complex procedures and components if the outer component is relatively thin and needs support to prevent its expansion. Formation of a mechanical joint by explosive requires much less explosive and thinner outer components can be utilized, but again, expensive outer supporting components and associated expensive handling is necessary when the outer component is not sufficiently thick to resist expansion by the explosion. Moreover the supporting inner surface of the supporting component must be shaped to conform to the exterior surface of the outer component and for complex surfaces this would increase the cost of the supporting component.
In the expansion process the explosive charge is placed coaxially within the bore of the portion of the tubular metal inner component to be expanded, and is usually contained within a component fabricated from a shock transmitting material such as polyethylene, located as a closely fitting insert within the tubular metal component.
Water is also often used as a means of transmitting shock waves being a most efficient transmitting medium giving a minimum attenuation of the shock wave. It is commonly used for explosive forming of components, but is has the disadvantage as a shock transmitting material for tube expansion that it can not be used to locate the explosive charge positively in the tubular component.
The method of explosive expansion of metal tubes hitherto used is impractical for the production of large explosively joined components in large numbers as is required in the production of oil-well drill pipes.
The bore size of a typical drill pipe is relatively large and thus requires a corresponding large shock transmitting insert to accommodate the explosive contained within. Polyethylene transmitting inserts are too expensive because of the volume of polyethylene required and the extended moulding cycle time necessary to maintain dimensional stability of the thick walled insert and prevent cavitation within the wall during manufacture. Moreover, the thick wall of the polythene insert would cause significant attenuation of the shock wave which, in turn, would require an increase in the explosive charge which would need to be accommodated by increasing the bore dimensions of insert thereby reducing the wall thickness. Thus a polyethylene insert of conventional design is not considered commercially acceptable.
The outside diameter and wall thickness of a typical drill pipe steel coupling are such that the coupling would require external support to prevent its radial expansion by the explosive forces needed to join it to a drill pipe. Closely fitting split dies of the kind previously used as external support for the outer component in explosively coupling tubular metal components are not suitable as they are expensive to produce, time consuming to assemble, and become distorted in use.